So what makes them always so happy to see you? Well, it's a combination of things but the main one is they are attached to you in most cases since birth and they love you.
What's important to know about dogs is they are very social animals and they feel the need to live in a group. That's why their ancestors lived in packs. Most dogs today still will not tolerate extremely longs periods of solitude. It drives them almost crazy.
In pack animals, their main social greeting is licking because it's like humans shaking hands. They also love hugs, touching, and being talked to. Dogs do understand you are not part of the pack but that you are different. Still, they hold a place in their brain and heart for humans.
You can relate a dog's happiness to see you as a toddler who has been at daycare all day. When mom or dad comes to pick them up they are so happy to see them. It's the same with dogs because they think of you as their parents.
They not only feel happy when they see you, they feel safer once you are home. In essence, dogs do practically worship their owners, and once you are home all is right with the world for them.
Scientists have done testing to see why dogs are always happy to see you and so far what they have discovered is this. While getting a brain scan the dog would be given something with the scent of the owner on it. The brain scan would show a dramatic change of excitement once the dog smelled his owner.
While you may never understand completely why your dog is always happy to see you, it doesn't really matter. What matters is that they make you feel happy, wanted, and loved. Nothing quite compares to dogs devotion!